This bug was filed specifically referencing the directory experimental/arm/hardened though all directories under the experimental branch which were checked exhibited the same problem. If possible please make the maintainers of other experimental profiles aware of this request.
Recently I noticed that http://distfiles.gentoo.org/releases/arm/autobuilds/current-stage3-arm64/ now contains a signed DIGESTS file. The file is dated the same as the uploads so I hope I didn't miss it the first time. I surely hope it wasn't generated and simply not uploaded.